White light emitting LEDs (“white LEDs”) are known and are a relatively recent innovation. It was not until LEDs emitting in the blue/ultraviolet part of the electromagnetic spectrum were developed that it became practical to develop white light sources based on LEDs. As taught, for example in U.S. Pat. No. 5,998,925, white LEDs include one or more phosphor materials, that is photo luminescent materials, which absorb a portion of the radiation emitted by the LED and re-emit light of a different color (wavelength).
Due to their long operating life expectancy (>50,000 hours) and high luminous efficacy (70 lumens per watt and higher) high brightness white LEDs are increasingly being used to replace conventional fluorescent, compact fluorescent and incandescent light sources.
Typically the phosphor material is mixed with a silicone or epoxy material and the mixture applied to the light emitting surface of the LED die. It is also known to provide the photo luminescent material as a layer on, or incorporate the phosphor material within, an optical component, a phosphor wavelength conversion component, that is located remotely to the LED die.
The color of the light generated by a light source, in particular LEDs, is determined predominantly by the device architecture and materials selection used to generate the light. As previously noted, many LEDs incorporate one or more phosphor materials, which are photo-luminescent materials, which absorb a portion of the radiation emitted by the LED chip/die and re-emit radiation of a different color (wavelength). This is the state of the art in the production of “white” LED light sources.
Typically, the LED chip or die generates blue light and the phosphor(s) absorb a percentage of the blue light and re-emits yellow light or a combination of green and red light, green and yellow light or yellow and red light. The portion of the blue light generated by the LED that is not absorbed by the phosphor is combined with the light emitted by the phosphor and provides light which appears to the human eye as being nearly white in color.
For these types of LED lights, the exact color of the resulting light is determined by the amount of blue light that is absorbed by the phosphor material in the wavelength conversion component, with more phosphor that is present resulting in more blue light that is absorbed. Therefore, as between any two LED lights having the exact same configuration, any variation in the amount of phosphor in the two lighting devices will cause a variation in the color emitted by these lighting devices.
The problem is that typical process control variations during the manufacture of the wavelength conversion component may very well result in differing amounts of photo-luminescent materials that is deposited within the wavelength conversion component. If the variation in photo-luminescent amounts is sufficiently large, this may result in visually perceptible differences between the light colors of lighting devices that include these wavelength conversion components. Such visually distinctive color differences are unacceptable for many commercial uses, particularly for the high-end lighting that often employ LED lighting devices.
Prior approaches to address this problem have generally proved unsatisfactory. For example, one possible solution is to perform color measurements on the wavelength conversion components after they have been manufactured. By performing these color measurements on each of the wavelength conversion components, one can determine whether the wavelength conversion component has a color that is within specification, which can then identified to be removed and discarded prior to its shipment to a customer.
The issue with this approach is that it requires individual color measurements for the wavelength conversion components, which can consume a significant and costly amount of time and effort on a per-component basis. This renders the approach unsuitable for most manufacturing environments that need to process the components on a large volume basis. Moreover, this approach can only be employed at the end of the manufacturing process, after the component has been manufactured. This delays identification of potential problems until very late in the manufacturing process which can be a significant problem, particularly for multi-stage manufacturing processes for multi-layered wavelength conversion components.
Therefore, there is a need for an improved manufacturing approach that reduces or eliminates perceptible manufacturing variations in the amount of phosphor that is deposited in a wavelength conversion component.